Semiconductor lasers are key components for use in fiber-optic communications and products such as CD players, CD ROM players and DVD players. Two different semiconductor laser structures are typically available for use in these applications: edge-emitting lasers and vertical-cavity surface-emitting lasers (VCSELs). The edge-emitting laser is the more established of the two semiconductor laser structures. Details of edge-emitting lasers and VCSELs are known in the art. However, to help an understanding of the invention, edge-emitting lasers and VCSELs are briefly described next.
An edge-emitting laser can be fabricated on a single-crystal substrate of semiconductor material such as InP or GaAs. Layers of various semiconductor materials are epitaxially grown on the substrate to form a layer structure that includes a p-i-n double heterostructure. The p-i-n double heterostructure has two principal functions. First, the p-i-n double heterostructure acts as part of an optical waveguide that extends in the plane of the layers constituting the double heterostructure. Second, an active region located in the intrinsic (i) layer of the p-i-n double heterostructure generates light in response to a forward electrical bias applied across the p-i-n double heterostructure. The light is generated by the recombination of holes and electrons injected into the active layer by the forward bias. The active layer typically includes one or more quantum wells.
Subsequent processing adds lateral waveguiding, current-confining structures and electrodes respectively directly or indirectly contacting the p-type and n-type layers of the p-i-n double heterostructure. Fabrication of the individual edge-emitting lasers is completed by cleaving the layer structure into individual die. The cleaving forms facets on the ends of each of the die. The facets are reflective and, together with the waveguide, form an optical cavity. The facets reflect a substantial fraction of the light generated in the active region back into the optical cavity. When optical gain provided by the active layer in the optical cavity exceeds the optical losses in the optical cavity, the semiconductor laser emits coherent light from the facets on the ends of the die. The light is emitted in a direction parallel to the major surface of the die.
VCSELs also include an active layer that generates light, but use a different structure to reflect the light back into the optical cavity. A VCSEL includes an optical cavity composed of an active region sandwiched between two distributed Bragg reflectors (DBRs). The DBRs and the active region are stacked on the substrate. The active region includes a p-i-n double heterostructure. Each of the DBRs is composed of multiple pairs of thin layers of materials having different refractive indices. The DBRs are highly reflective in a narrow wavelength band defined by the refractive indices and thicknesses of the layers. The materials of the DBRs are typically semiconductors or dielectrics. Current injected through a current-confining structure into a narrow region of the active region generates light. The light is emitted through one of the DBRs in a direction normal to the major surface of the substrate. For single transverse-mode VCSELs, the mode diameter is only a few microns and the divergence of the beam is relatively small.
Edge-emitting lasers and VCSELs each have their own advantages and disadvantages. Edge-emitting lasers have a much higher single-pass optical gain as a result of their longer optical cavity. This makes it possible for edge-emitting lasers to be fabricated from a wider range of materials and to generate light over a wider range of wavelengths. Edge-emitting lasers also generally have a better high-temperature behavior. Further, edge-emitting lasers have a much higher single-mode power capability stemming from the larger volume of the optical cavity.
VCSELs have the advantage that they can be tested in wafer form since they do not have to be cleaved to make a laser. VCSELs are typically smaller than edge-emitting lasers. Consequently, they typically require a lower drive current to generate a moderate level of optical power, e.g., 1 milliwatt. This makes VCSELs less expensive to use, since high-current laser drivers are expensive and have high power consumption when operated at high modulation rate. Moreover, the smaller size of VCSELs means that more VCSELs can be made on each wafer and, hence, a lower cost per VCSEL. VCSELs have an optical mode size larger than that of edge-emitting lasers and the optical mode is a better match to single-mode fiber. This makes it easier and cheaper to couple light from the VCSEL to the fiber.
Thus, VCSELs have many advantages, but edge-emitting lasers remain a better solution for higher power applications. What is needed, therefore, is a surface-emitting laser with better high-power characteristics than conventional VCSELs.